control of heat release rate in s.i. engines p m v subbarao professor mechanical engineering...
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Control of Heat Release Rate in S.I. Engines
P M V SubbaraoProfessor
Mechanical Engineering Department
Development of Models to Design & Select Hardware…...
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Mass of Fuel per Cycle
• Accurate thermodynamic analysis of engine cycle determines the fuel to be inducted per cycle.
• This analysis cannot advocate the rate of consumption of this fuel mass with in the cycle.
• The pace and duration of fuel consumption is an essential knowledge in developing low emission and high fuel economy vehicular engines.
• Many hardware (mechatronic ) solutions along with software solutions are being continuously developed.
• Development of these solutions need a knowledge of fuel consumption vs crank angle.
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Quasi-static Approach to Rapid Combustion
For an infinitesimal displacement of crank, d
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Cumulative Fuel Consumption Model
• The most common method of defining cumulative combustion is with a mass fraction burned curve.
LHVm
dd
Q
MFBcombtotalf
gen
soc
,
•Mass fraction burned is the ratio of the cumulative heat release to the total heat release.• Therefore if the mass fraction burned is known as a function of crank angle, then the apparent heat release can be approximated.•When the cumulative mass fraction burned is graphed against crank angle degrees, the curve can be described using a Wiebe function
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The Wiebe function
• θ is crank angle degrees with θsoc corresponding to the initialization of heat release and
• Δθ corresponding to the duration of burn.
• The equation is also defined by two constants a and m which have typical values of 5 and 3 respectively.
1
exp1m
socaMFB
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The Actual Release of Net Heat Rate in SI Engines
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Phases of Combustion in Homogeneous SI Engine
Crank Angle,
Sparking
Start of Combustion
End of Combustion
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Mixture Burn Time vs Engine Speed
The time for an overall burn is:
36060
%90%90
revkN
to
General acceptable value is 50o crank angle for the overall burn
N (rpm) t90%(ms)
Standard car at idle 500 16.7
Standard car at max power 4,000 2.1
Formula car at max power 19,000 0.4
Note: To achieve such high engine speeds a formula car engine has a very short stroke and large bore.
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Phases of Combustion Vs Generation of Air-fuel Mixture : SI Engines
• For spark ignition engines, Mixture preparation precedes all the other engine processes.
• Metering the ambient air and fuel and forming a mixture that satisfies the requirements of the engine over its entire operating regime is pre-decided.
• The structures of fuel spray dominates the mixture preparation process.
• This also strongly affect the durations of combustion phases over a wide range of operating conditions.